Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle the information being accessed by means of read/write heads generally located on a pivoting arm which moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle and the other near the bottom. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above is prone, however, to several short comings. First is the problem of vibration generated by the balls rolling on the raceways. Ballbearings used in hard disk drive spindles run under conditions that generally guarantee a physical contact between raceways and balls, this in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally even and smooth, but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfections in sphericity in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. This source of vibration limits therefore the datatrack density and the overall performance of the disc drive system.
Another problem is related to the application of hard disk drives in portable computer equipment and the resulting requirements in shock resistance. Shocks create relative acceleration between the disks and the drive casting which in turn shows up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material and leave permanent deformation and damage on raceways and balls.
Moreover, mechanical bearings are not always scalable to smaller dimensions. This is a significant draw back since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluid--either gas or liquid--functions as the actual bearing surface between a stationary base or housing and the rotating spindle or rotating hub and the stationary surrounding portion of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids, or even air have been utilized for use in hydrodynamic bearing systems. The reason for the popularity of the use of air, is the importance of avoiding the outgassing of contaminants into the sealed area of the head disc housing. However, air does not provide the lubricating qualities of oil. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance.
Thus, in the case of a hydrodynamic bearing employing a liquid lubricant, the lubricating fluid and its components must be sealed within the bearing to avoid loss of lubricant which results in reduced bearing load capacity. Otherwise, the physical surfaces of the spindle and housing could contact one another, leading to increased wear and eventual failure of the bearing system. Equally seriously, loss of a seal or failure to control the fluid level within the bearing system could cause contamination of the hard disk drive with lubricant particles and droplets as well as outgassing-related condensation.
It is a further important problem that start-stop cycles and load exceeding the bearing capacity result in wear-and-tear of the bearing surface. This creates particles which are free to move in the lubrication film and which will act as slurry, thus accelerating said wear-and-tear.
A further difficulty with prior art designs of liquid lubrication hydrodynamic bearings is that frequently voids or gas bubbles may occur in the bearing area thereby reducing the effective bearing surface and the related load capacity.
Yet another difficulty of known hydrodynamic bearing designs is their sensitivity to both machining tolerances and the temperature ranges across which they must be operable. Both of these issues are critical in hydrodynamic bearings because of the very narrow gaps between the rotating and stationary parts which must be maintained so that the fluid is effective in lubricating the bearing surfaces, while the tolerance between the bearing surfaces is not so great as to allow tilting of the rotating disks supported by the rotating spindle motor out of the horizontal plane. Thus, it is clear that a number of considerations must be balanced in designing an effective hydrodynamic bearing spindle motor for use in a disk drive.